Improvement in the quality of artificial illumination is today a top priority requirement. In fact, there are increasingly more circumstances in which man finds himself spending a large part of his life in artificial illumination conditions. This is due to the constructional features of many industrial spaces, hospitals, department stores, underground railways, airports and the like, whose indoor areas are not exposed to direct skylight and sunlight. Moreover, in various regions of the planet, the conditions of low temperature (for example in Canada) or, vice versa, high temperature and humidity (for example in Singapore), which characterize lengthy periods of the year, encourage more and more development of underground urban planning, as it is much easier to achieve satisfactory climate control underground. Finally, the quality of artificial illumination has a considerable impact on the quality of life for populations living at high latitudes, where there is little or no sunlight for lengthy periods of the year.
On the other hand, the energy question today places the need to reduce power consumption used for illumination at the forefront. As can be seen from recent legislation, this need provides for the elimination, within a few years, of conventional incandescent lighting, which produces a black body emission spectrum similar to sunlight, but which dissipates most of the energy in heat, in favor of new technologies, such as LEDs and laser diodes. LED technology, already widely used for backlighting screens and panels, in road signs and in motor vehicles, is today preparing to enter the market of indoor and outdoor lighting. One of the main difficulties is in this case constituted by the quality of the lighting, above all for low cost types of sources, which exhibit lower consumption. This is the case, for example of InGaN—GaN LEDs emitting in the blue region (at 430-470 nm) completed by the presence of a phosphor which emits broad-band radiation in the yellow region (around 580 nm). These sources have a spectral profile differing substantially from that of a black body, presenting a peak of maximum intensity at the emission wavelength of the LED, and a second peak of lesser intensity at the maximum phosphor emission efficiency. The difficulty linked to this type of source is related both to the very high color temperature (≈7000K), as described in U.S. Pat. No. 7,259,400 which is incorporated by reference, which gives the light the characteristic bluish color, and to the lack of green and red components in the spectrum. Although this lack is not noticed when illuminating a white object, given that the yellow component produced by the phosphor excites, in a balanced manner, the cones in the eye sensitive to red and green, it becomes important when illuminating colored environments, given that green or red objects appear dark.
Within the scope of current technological development, the majority of efforts aimed at improving the quality of illumination are concentrated on the spectral characteristics of the light produced having the object of making it perceived to be as close as possible to sunlight. Within the context of the definition above, this approach nonetheless does not include the aforesaid fundamental aspect characterizing natural illumination, namely the presence in nature of not one but two different light sources: the sky and the sun. The effect can be understood by considering the different CCT (Correlated Color Temperature) of the two sources, defined as Planck radiator temperature (black body radiation), which is perceived by the eye as a color closer to that of the source in question. If we consider, for example, natural illumination in the late afternoon, when the sky, almost as luminous as the sun, has a CCT of over 9000K, and the sun has a CCT of under 4000K, it is evident that the spectrum resulting from the sum of the two sources is nowhere near a black body spectrum. Nonetheless, this type of illumination is extremely pleasing to the eye. Therefore, the presence of this particular dichromatism, associated with direct and scattered light, is an important element, not considered previously, to be added to the previous ones in order to assess the quality and pleasantness of artificially created illumination.
It is also important to note that an illumination method based on a single type of source can at the most simulate, in the case of a spectral profile similar to that of the sun, “lunar” illumination. In this context, as the shadows are very dark, they are not pleasing. For this reason, artificial illumination often uses many sources, or reflections on walls or ceiling, to minimize shadows.
A first proposal for artificial illumination based on indoor reconstruction of natural illumination as composed of sky and sun was presented in a work exhibited in various science and art exhibitions, also presented at the Genoa Science Festival in 2003 and 2005 and in Vilnius Railway Station (Lithuania) in 2007 (www.diluceinluce.eu, which is incorporated by reference). In these contexts various installations and various experimental apparatus were produced, including “indoor” reconstruction of the sky, i.e. reconstruction of the Rayleigh scattering process caused by nanometrical density fluctuations of a transparent medium which, in the case of the atmosphere, determines the light and color of the sky and of the sun. As scattering medium an aqueous dispersion of silica nanoparticles, with diameter of around 20 nm, was used. This dispersion, presenting refractive index fluctuations of considerable amplitude (approximately 15%) on scale lengths below 1/10 of the wavelength, allowed the production of a good diffuser operating in Rayleigh regime. At the maximum concentrations used, i.e. for a silica volume fraction of 2% of the dispersion volume, it proved capable of producing, on a beam of light passing through it for a few meters, the same color variation which, in the atmosphere, requires hundreds of kilometers of distance. The dispersion thus produced was placed in transparent PMMA containers for containment. “White” light sources were then used to simulate the sun, namely halogen lamps with calibration filters or mercury vapor discharge lamps. By using different dispersion concentrations, different sky volumes, different installation geometries, comprising combinations of containers of different shape and dimension, different positions of sky and sun, and the presence of absorbent or reflective screens to simulate clouds, spectacular reconstructions were obtained of light effects due to the presence of the sky and the sun at different times of day.
However, the dispersion of nanoparticles in water presents numerous problems that make its use in the lighting sphere almost impossible. In fact, due to the different specific weight Ps, between water and nanoparticles, which typically increases with the refractive index value of the nanoparticles, n1, (e.g. Ps=2.2 g/cm3 and n1=1.5 for SiO2, while Ps=4.23 g/cm3 and n1=2.7 for TiO2), the nanoparticles tend to deposit through gravity on the bottom of the container, as they are held in suspension only through Brownian motion. For this reason the suspension must be stirred periodically. For the same reason the concentration of nanoparticles is not constant, but decreases with height. The problem may be reduced, but not eliminated, using nanoparticles of extremely small diameter, in order to maximize the effect of Brownian motion. In this case, however, the scattering efficiency is influenced negatively, a fact that implies the use of diffusers of very high depths (at least in the order of tens of cm). Moreover, the suspension in liquid is somewhat unstable from a bacteriological viewpoint, especially if continuously exposed to light. It then presents the risk of freezing, which prevents its use for outdoor installations. Moreover, the liquid medium presents the problem of containment, which is important in the case of diffusers of medium-large dimensions, and the need to combat pressure due to the height of the liquid, which implies the use of containers produced in thick material (several cm) in the case of heights of over one linear meter.
U.S. Pat. No. 6,791,259 B1, which is incorporated by reference, describes a white light illumination system comprising an LED or laser diode, a light diffuser material and a phosphor or luminescent dye material. The diffuser material preferably comprises particles dispersed in a substrate. The particles that scatter light have a diameter between 50 and 500 nm, preferably a diameter between λ/3 and λ/2, where A is the wavelength of the emission peak of the radiation source. In this application, however, the color nanodiffuser is integrated at the level of the active element of the source, that is, it is positioned either before the phosphor or in the phosphor, in order to scatter preferably the blue component produced by the LED or laser diode, otherwise with low divergence, and to uniform it with the yellow component scattered by the phosphor, instead produced with a wide angle of divergence. The fact that the two yellow and blue components are scattered from practically coincident diffuser centers is a necessary condition to remove the “halo” phenomenon, characterized by the presence of a dominant blue color in the direction of maximum emission, and of a dominant yellow color in the peripheral area of the light cone produced by the source, that is, to uniform color distribution of the radiation at different angles.
WO 02/089175, which is incorporated by reference, describes light sources based on UV with reduced dispersion of UV radiation. The light sources are LEDs which emit in the UV and which are combined with UV reflectors constituted by particles dispersed in a solid material transparent to visible light. A phosphorescent material is applied to the UV source to convert UV radiation into visible light. In a particular embodiment the phosphorescent material is applied to the surface of the UV LED and a layer of diffuser material is applied to the phosphorescent layer. The aim of this illumination device structure is to reduce the amount of UV radiation not converted into visible light and does not tackle the problem of reproducing a light similar to natural light produced by the sun and the sky.